1. Field of the Invention
The present invention relates to an ultra-low temperature alloy, and more particularly to a new Fe--Mn--Al--C--Nb--Si--Cu alloy which can be made by adding Nb, Si and Cu as minor alloying elements to a conventional Fe--Mn--Al--C alloy steel. The present invention also relates to an ultra-low temperature alloy, provided by controlled rolling the alloy. The invention further relates to a process of forming the alloy, including a controlled rolling of the alloy. The alloy in accordance with the invention has greater impact toughness and strength than a standard ASTM A553 9% nickel steel for use in ultra-low temperature materials.
Presently, the demands for materials for liquefied natural gas (LNG) storage tanks are increasing because of the economics thereof. Consequently, demands for the aforementioned 9% nickel steel, which has high strength and toughness even at -196.degree. C. have increased considerably. However, this conventional alloy steel contains a large amount of nickel, which is an expensive strategic material; therefore, the price of the steel fluctuates very widely in the market. In addition, since the 9% nickel steel has a body centered cubic lattice structure, its mechanical properties tend to degrade rapidly at about -190.degree. C.
2. Description of the Prior Art
Toughness is an important factor desired in low temperature materials. In order to improve the toughness at low temperatures, it has been recommended that a steel be stabilized to have an austenitic structure with face-centered cubic lattices without a ductile-brittle transition temperature. The Fe--Mn--Al--C alloy mentioned above was developed for this purpose, J. Charles, et al., Met. Prog. 119, 71 (1981). The Fe--Mn--Al--C alloy with an austenitic structure is superior in toughness but inferior in strength to the 9% nickel alloy steel.